mmc: bcm2835: fix delays in bug workaround

The back-to-back-writes workaround in the BCM2835 MMC driver assumed
that get_timer() returned uS. Now that it returns mS, the delay is far
too long. Use udelay() directly to avoid this. Dispense with the
"last_write" code since we now have no way of recording an absolute
time in uS. The difference between two un-averaged tests loading a
zImage is 445 mS vs the original 412 mS, so the difference doesn't
appear too relevant.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>